Experimental study on the channel effect in emulsion explosives

Journal of Materials Processing Technology 85 (1999) 25 – 29

Experimental study on the channel effect in emulsion explosives Fumihiko Sumiya a,*, Kazushi Tokita a, Masashi Nakano a, Yuji Ogata b, Yuji Wada b, Masahiro Seto b, Kunihisa Katsuyama b, Shigeru Itoh c b

a Hokkaido NOF Corporation, Koshunai 549, Bibai, Hokkaido 079 -0167, Japan National Institute for Resources and En6ironment (NIRE), 16 -3 Onogawa, Tsukuba, Ibaraki 305 -8569, Japan c Faculty of Engineering, Kumamoto Uni6ersity, 2 -39 -1 Kurokami, Kumamoto 860 -0862, Japan

Abstract The precursor air shock wave (PAS), which propagates ahead of the detonation front in an air channel, precompresses and desensitizes the unreacted explosive charges. Under some conditions, the PAS causes detonation failure. This phenomenon is known as the channel effect. To investigate the mechanism of the channel effect in emulsion explosives, some experimental work has been carried out using a high-speed framing camera. The results of photographic observation demonstrated that the difference between PAS velocity and detonation velocity was the primary factor for the channel effect. It is assumed that a decrease in the PAS velocity can prevent the channel effect. The ability of increased surface roughness of the inner wall to decrease the PAS velocity was tested. Some experiments were conducted to investigate the effects of surface roughness on the PAS velocity and the detonation propagation. The experimental results indicate that increased surface roughness can reduce the PAS velocity and prevent detonation failure. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Channel effect; Precursor air shock wave; High-speed framing camera; Emulsion explosive; Detonation failure; Surface roughness

1. Introduction When an explosive is detonated in a borehole, a shock wave is generated in the air channel between the explosive charge and the inner wall of borehole in smooth blasting. As the velocity of a shock wave propagating in an air channel is higher than that of a detonation wave under some conditions, the shock wave travels ahead of the detonation wave. This precursor air shock wave (PAS) propagating ahead of the detonation wave precompresses the unreacted explosive and desensitizes the explosive charge. When the detonation wave reaches this precompressed point, the detonation propagation ceases to be regular. Thus, a decrease in detonation velocity and detonation failure occurs. This phenomenon is well-known as the channel effect. Several works on the channel effect have been conducted under various conditions, using commercial explosives [1–4], while visual studies on the channel effect have been reported by a few researchers [5,6]. To clarify * Corresponding author. Tel.: + 81 1266 72211; fax: +81 1266 21114.

the cause of the channel effect, it is necessary to elucidate detonation propagation in explosive charges and progress of the PAS in the air channel. In order to study the mechanism of the channel effect, we carried out photographic observation using a high-speed framing camera. Our investigations were conducted taking the following into consideration: (i) the influence of geometric conditions on the propagation of the detonation and the PAS; (ii) the effect of surface roughness on detonation and PAS propagation.

2. Experimental

2.1. Sample explosi6e Typical water in oil emulsion explosives were used in these experiments. The emulsion matrix consisted of 84% oxidizing agent, 11% water and 5% fuel oil and emulsifier. Resin microbubble (RMB) was added as sensitizer to the emulsion matrix to adjust the explosive initial density to 1000 kg m − 3. Detonation velocities of sample explosives were controlled in the range 3000– 3300 m s − 1.

0924-0136/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00248-9

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F. Sumiya et al. / Journal of Materials Processing Technology 85 (1999) 25–29

Fig. 1. Experimental setup used in test series A and B.

2.2. Experimental setup

3. Results and discussion

The emulsion explosive was charged in a transparent tube with rectangular cross section, which was made of 4 mm thick polymethylmethacrylate (PMMA) plates. The length of the rectangular PMMA tube was 1000 mm. The inner diameter of the tube was maintained at 14 mm, but the inner height was changed to modify the tube geometry. The concept of the ’decoupling coefficient’ in this paper is defined as the ratio of the inner wall height of the rectangular PMMA tube to the explosive charge height. The explosive charge height in the experiment was set at 14 mm. The sample emulsion explosive was put into a shape of 14 × 14 mm − 2 in cross section and 800 mm in length. PMMA tubes without any surface treatment were used for test series A. Fig. 1 shows the experimental setup. In test series B, sandpaper was glued onto the upper inner wall of the PMMA tube and the surface roughness of the sandpaper was changed to study the effects of surface roughness on the propagation of the PAS. The experimental arrangement for test series B was the same as previously described for test series A.

The PAS, propagating faster than the detonation wave, was observed in the experiments using a rectangular PMMA tube. The PAS emitted high intensity visible light over 100–200 mm length. As the PAS was self-luminous, the PAS front could not be identified. However, no PAS was observed in the experiments using a PMMA tube without its upper wall. The detonation velocity was steady, within about 150 m s − 1 along the whole length of the rectangular PMMA tube, except in the case in which detonation failure occurred.

2.3. Measuring system Propagation of the detonation wave and PAS was observed using a high-speed framing camera (Cordin model 124). This camera can take 26 pictures at a frame speed of 1.0 × 105 –2.5 × 106 frames per second (FPS). In these experiments, the frame speed of 1.0 ×105 – 5.0× 105 FPS was used and the interframe time was 2 – 10 ms. A xenon flash was used as a light source. The positions of the PAS and the detonation wave were determined from high-speed framing photographs. The electric detonator was controlled within 1 ms accuracy for initiation and synchronized with the framing camera and the xenon flash. Photographic observations were conducted from the side face of the PMMA tube.

3.1. Influence of the geometric conditions on the propagation of the detonation and the PAS Experimental results from test series A are shown in Table 1. The phenomenon of the detonation failure, i.e. channel effect, were observed in experiment nos. A-2– A-7, whose decoupling coefficient is ranged within 1.21–2.71. In these experiments, the detonation propagation improved with increasing decoupling coefficient and with decreasing PAS velocity. Johansson et al. [1] proposed that detonation failure occurs in the range of decoupling coefficient 1.12–3.82 in the case of dynamite of charge diameter 25 mm. Liu et al. [4] described that for an emulsion explosive of charge diameter 25 mm, the detonation failure occurs in the range of decoupling coefficient 1.28–3.56. These experimental results present good agreement with the results obtained by Johansson et al. and Liu et al., considering the difference of charge configuration. It is considered that the range of the detonation failure depends on both the decoupling coefficients and the explosive diameter. When the decoupling coefficients is sufficiently large, the PAS scarcely travels ahead of the detonation wave, therefore not precompressing the explosive charge. In addition, the channel effect is a characteristic of an explosive charge with a small diameter close to its critical diameter. When the

800 410 400 500 490 580 610 800 800 800 800

A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-10 A-11

3200 3080 3130 3170 3120 3130 3180 3110 3100 3130 3100

Detonation velocity, X (m s−1) 3200 3860 4060 4020 3900 3800 3840 3550 3450 3430 3290

PAS velocity, Y (m s−1) 1.00 1.28 1.30 1.27 1.25 1.21 1.21 1.14 1.11 1.10 1.06

Velocity ratio, Y/X — 180 210 170 180 180 180 — — — —

Critical compressed length (mm)

Critical compressed length: travelling length of the PAS at the detonation failure; propagated length of detonation at the detonation failure. Critical compressed time: time from initiation to detonation failure; time when the PAS reaches the point of the detonation failure.

1.00 1.21 1.50 1.79 2.00 2.29 2.71 3.00 4.00 5.00 7.00

Propagation length (mm)

Experimental no. Decoupling coefficient (h/14)

Table 1 Experimental results in test series A

— 60 60 50 50 50 50 — — — —

Critical compressed time (ms)

F. Sumiya et al. / Journal of Materials Processing Technology 85 (1999) 25–29 27

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F. Sumiya et al. / Journal of Materials Processing Technology 85 (1999) 25–29

Fig. 2. Relationship between the propagation length and time of the PAS.

explosive diameter is large enough, the PAS will not be able to compress and desensitize the explosive. When the ratio of the PAS velocity to the detonation velocity became large, the length of the unreacted explosive charge compressed by the PAS became important. As a result, the time during which the unreacted explosive was compressed by the PAS became long enough to cause detonation failure. We defined the critical compressed length and critical compressed time as follows. Critical compressed length: travelling length of the PAS at the detonation failure; propagated length of detonation at the detonation failure (Table 1). Critical compressed time: time from initiation to detonation failure; time when the PAS reaches the point of the detonation failure (Table 1). From experimental results in Table 1, we concluded that the critical compressed time is 50 – 60 ms and the critical compressed length is 200 mm. Bjarnholt et al. [5] proposed a critical compressed time of about 90 ms in their experiments using the Gurit explosive, which is a powder explosive sensitized by NG/EGDN and inert loading with SiO2. Their results agree well with the results of this study.

3.2. Effect of surface roughness on detonation propagation and PAS propagation Regarding the detonation propagation of explosives in tube, it is known from experience that detonation propagation is improved when the wall surface is rough. Suzuki et al. [7] demonstrated that the structure of a reflected shock wave over a wedge with surface roughness is unsteady. Therefore, we assumed that increased surface roughness induces shock waves to reflect irregularly at the upper wall and prevents the PAS from propagating smoothly in an air channel

because of wall friction. Thus, the purpose of increasing roughness is to decrease PAS velocity. The sandpaper was therefore glued onto the inner upper wall to increase the roughness. Particularly high-speed framing photographs were taken under two different sets of conditions: one in a PMMA tube with sandpaper of mesh c 80 and the other in a PMMA tube without sandpaper. It was observed that the positions of the detonation front were identical in both case, but the position of the PAS in the case of the PMMA tube without sandpaper was much farther ahead of the detonation front than it in the case of the PMMA tube with sandpaper. Detonation failure occurred in the case of the PMMA tube without sandpaper. However, in the case of the PMMA tube with sandpaper, the detonation failure did not occur and the detonation wave propagated completely. Fig. 2 presents the relationship of propagation length versus time of the PAS determined from the sequence of high-speed framing photographs. It is shown that the addition of sandpaper reduces the propagation velocity of the PAS and detonation failure is prevented. Experiments were performed in the case of various decoupling coefficients, using sandpaper with various meshes. Table 2 summarizes the experimental results obtained in test series B. In the case of PMMA tubes without sandpaper, detonation failure was observed when the decoupling coefficient was in the range of 1.21–2.71, as previously described in Table 1. The experimental results indicate that the propagation distance of the detonation wave becomes longer as the surface roughness of tube wall is increased. From Table 2, it can be seen that the phenomenon of detonation failure will appear when the ratio of the PAS velocity to the detonation velocity becomes greater than 1.21. This result indicates that detonation failure occurs when the

F. Sumiya et al. / Journal of Materials Processing Technology 85 (1999) 25–29

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Table 2 Experimental results in test series B Decoupling coefficient

Sandpaper rough- Propagation length ness (mm)

Detonation velocity, X (m s−1)

PAS velocity, Y (m s−1)

Velocity ratio, Y/X

1.21 1.21 1.21 1.21 1.21 1.21

Nothing c600 c400 c220 c150 c80

410 430 430 800 800 800

3080 3010 3060 3120 3030 3190

3860 3730 3760 3740 3200 3250

1.28 1.24 1.23 1.20 1.06 1.02

1.50 1.50 1.50 1.50 1.50 1.50

Nothing c600 c400 c220 c150 c80

400 490 470 530 800 800

3130 3180 3150 3120 3120 3170

4060 4010 3910 3810 3300 3230

1.30 1.26 1.24 1.22 1.06 1.02

1.79 1.79 1.79 1.79 1.79 1.79

Nothing c600 c400 c220 c150 c80

500 600 590 630 800 800

3170 3180 3080 3070 3260 3200

4020 3940 3830 3750 3360 3300

1.27 1.24 1.24 1.22 1.03 1.03

2.29 2.29 2.29 2.29 2.29 2.29

Nothing c600 c400 c220 c150 c80

580 610 630 690 800 800

3130 3080 3050 3010 3130 3230

3800 3730 3720 3650 3350 3290

1.21 1.21 1.22 1.21 1.07 1.02

critical compressed time of unreacted explosive by the PAS is 50–60 ms. These results are in good agreement with those obtained in test series A.

wish to express my sincere thanks to Dr Kato; Research and Development Section Director and Mr Hirosaki for reviewing the manuscript.

4. Conclusions

References

The following conclusions were obtained in this research. (1) The geometric difference of tube into which the explosive is placed has important influences on the values of the decoupling coefficients for occurrence of channel effect. (2) When the precompression time, the time lag between the PAS and detonation front, reaches its threshold value, the channel effect occurs. (3) The surface roughness of tube wall has an inhibitive influence on the occurrence of channel effect in emulsion explosive. It is good for detonation propagation.

Acknowledgements These investigations were performed in the Research and Development Section of the NOF Corporation, Taketoyo Plant, where this author worked previously. I

[1] C.H. Johansson, H.L. Selberg, A. Persson, T. Sjolin, Channel effect in detonation in tubes with an open space between the charge and the tube wall, Preprint from XXXI Int. Cong. of Industrial Chemistry, Liege, Belgium, 1958. [2] M. Nakano, N. Mori, The study of the propagation of detonation in bore holes, J. Ind. Explos. Soc. Jpn. 40 (4) (1979) 245–257. [3] K. Hanasaki, Y. Muneta, H. Sakai, N. Mori, Behaviour of compressed air in channel of charged hole, J. Ind. Explos. Soc. Jpn. 45 (3) (1984) 149 – 156. [4] Q. Liu, A. Bauer, R. Heater, The channel effect for AN/FO, slurries and emulsion, Proc. 4th Mini-Symp. on Explosives and Blasting Research, International Society of Explosives Engineers, Anaheim, CA, 1988, pp. 33 – 48. [5] G. Bjarnholt, U. Smedberg, On detonation driven air shocks in the air gap between a charge and its confinement, Proc. 8th Int. Symp. on Detonation, Naval Surface Weapons Center, Albuquerque, NM, 1985, pp. 1069 – 1079. [6] U. Smedberg, M. Edvardson, High speed photography and computer modeling of the initial part of the channel effect, Swedish Detonic Research Foundation (SveDeFo) Report, DS-1987-10, 1987. [7] T. Suzuki, T. Adachi, S. Kobayashi, Mach reflection over a wedge with surface roughness in a shock tube, Proc. 1989 Natl. Symp. on Shock Wave Phenomena, ISAS, Sagamihara, Japan, 1989, pp. 21 – 29.